Allison and Gerard Baden-Clay pictured on their wedding day. Allison disappeared April 20, 2012. The body of the mother-of-three was found 10 days later on a Brisbane creek bank

Gerard Baden-Clay told his mistress Toni McHugh the night before his wife Allison Baden-Clay went missing that he was going to leave his wife and that he and Ms McHugh would have a future together, a court heard on Tuesday.

Crown Prosecutor Todd Fuller QC told the court that emails between Baden-Clay and Ms McHugh, who he'd nicknamed 'GG' meaning gorgeous girl, told the story of their relationship and the fact he no longer loved his wife.

Police who investigated the death of Allison are expected to take the stand at her husband's murder trial on Wednesday.

Earlier on Tuesday, the first day of Baden-Clay's long-awaited murder trial, the 43-year-old stood in the dock in the Brisbane Supreme Court and pleaded not guilty to the charge of murdering Allison in April 2012.

The prosecution opened its case against the former real estate agent and the court heard Baden-Clay and Ms McHugh had set a date - July 1, 2012 - when they would be together.

'They started planning a life together,' Mr Fuller QC told the court. 'He planned to leave his wife who he no longer loved.'

The opening day of his trial on Tuesday saw Baden-Clay wiping his eyes as graphic images of his wife's decomposed body were shown to the jury.

One photo showed the body of a long-haired woman lying on her side with her arms splayed, dressed in what appears to be exercise clothes.

Allison's badly decomposed body was found on a bank near a bridge on Brisbane's Kholo Creek

Scene pictures submitted in court capture detectives gathering near the place where Allison's body was discovered

Allison's body was discovered by a kayaker on April 30, 2012, 10 days after she was reported missing

Prosecutors in the case of Gerard Baden-Clay, Todd Fuller, (left) and Danny Boyle (right), arrive at the Supreme Court in Brisbane on Tuesday

Senior Sergeant Ewan Taylor, who attended the scene where Allison's body was found, showed photographs of the river bank showing indentations made by the victim's body, as well as pictures of her body in situ on the mud bank.

BADEN-CLAY AND MS MCHUGH WORKED TOGETHER AT HIS REAL ESTATE BUSINESS

Mr Fuller QC said Ms McHugh had worked with Baden-Clay at his real estate business in western Brisbane from 2007, and that she'd had an affair with him. But the affair and Ms McHugh's employment at the business ended when Allison learned of the relationship.

Allison then went to work with her husband at the business.

When Baden-Clay and Ms McHugh had resumed their romance and she learned she was going to be at the same real estate conference on April 20, 2012 that Allison was attending she urged him to reveal their affair.

Allison Baden-Clay's (left) body was discovered by a Brisbane kayaker on April 30, 2012. The discovery sparked a massive police response (right)

'[Toni McHugh] said he needed to tell his wife in no uncertain terms she would be at the conference,' Mr Fuller told the court.

'As of the 19th of April he wanted to leave his wife and be with Toni McHugh.

'He was under . . . Significant relationship pressure.

'When these two parts of his life came together previously there had been significant fall out.

'He had to end the relationship and he had to end [Ms McHugh's] employment.

'These two women were about to come together on the 20th of April.'

Mr Fuller said Baden-Clay had also been under serious financial pressure and had tried to borrow $400,000 from local politician, Dr Bruce Flegg.

He owed money to friends, couldn't pay the interest on loans and Allison had two insurance policies in her name and a further policy on her superannuation scheme, the court was told.

ALLISON BADEN-CLAY HAD UNDERGONE MARRIAGE COUNSELING

The court heard that since 2003 Allison had sought psychiatric help for anxiety following the birth of her first daughter and her next two children and had been prescribed anti-depressants.She had also seen a psychologist and Relationships Australia counselor Carmel Ritchie about the state of her marriage.
Allison and Gerard Baden-Clay pictured in an undated photograph

She told one psychologist about the stress she was under because of her marriage and financial problems.

She had become aware in October 2011 her husband was having an affair and told Ms Ritchie the month before her death of her 'feelings of inadequacy and guilt'.

On her last session with Ms Ritchie, on April 16 2012, she was accompanied by Baden-Clay, and they agreed to a system where every second night for ten to 15 minutes he would listen to her describe her feelings.

But the court heard Baden-Clay had already made up with his mistress Toni McHugh.

'They started planning a life together,' Mr Fuller QC told the court.

'He planned to leave his wife who he no longer loved.'

He said the lovers had set a date - July 1, 2012 - when they would be together.

COURT SHOWN PICTURES OF ALLISON'S CAR

Mr Fuller showed photographs of Allison's Holden Captiva, which he said her husband had used to transport her body on the evening of April 19, 2012 and into the morning of April 20 from their Brookfield home in Western Brisbane 14km away to Kholo Creek on the Brisbane River.

He said the car had tested positive for blood which DNA testing later proved was Allison's.

Baskets of toys had been placed in the back of the car before police inspected the vehicle, but upon interviewing the Baden-Clay's daughters they could not remember seeing the toys - which were old and they thought had been given to charity - since the previous Christmas.

He said a kayaker had found her badly decomposed body, which was swollen and discoloured, 10 days later on a steep bank of the creek below Kholo Bridge.

Allison's parents Geoff and Priscilla Dickie were not present at court

Mr Fuller told the jury they would hear from neighbours of the Baden-Clays who had heard a male and female arguing, and at least two neighbours who had heard a woman scream.

They would also hear from neighbours in the Kholo Creek area, one of whom heard two thuds in the early hours of April 20.

Mr Fuller QC said forensic evidence would be presented at court showing that six plant samples were found entwined in Allison's hair.

He said while two of the plants were consistent with vegetation at the site where her body was found, all six plants were growing on the patio or in the yard of the Baden-Clays' home.

He said her body had been taken to Kholo Creek to 'hide the truth' via its decomposition.

He said the blood in Allison's car was a 'transfer stain' from a wound suffered by Allison, although decomposition had meant no exact cause of death had been established.

The coroner had found two injuries, bruising to her chest and a chipped tooth, but could not say exactly when they had occurred.

An autopsy in May 2013 showed her left arm had been bent up behind her head, her right arm was bent at the elbow and her jumper was inside out and showed signs consistent with her body being dragged.

On Tuesday the jury was shown a graphic photo of Allison's body where it was found on a creek bank in Brisbane

FAMILY STAND BY BADEN-CLAY

The family of Baden-Clay rallied around the former real estate agent on the first day of his trial.

Dressed in a dark suit and still wearing his wedding ring, the 43-year-old appeared relaxed as he shared a few brief words with his sister and parents during a break in proceedings on Tuesday.

Earlier the father-of-three stood in the dock and said firmly and loudly, 'not guilty, Your Honour' when asked to enter a plea to the charge of murdering his wife.

When he entered the glass panelled dock he smiled at his parents Nigel and Elaine Baden-Clay, who sat directly behind him in the packed public gallery with their daughter Olivia Walton and other supporters.

Allison's parents Geoff and Priscilla Dickie were not present at court, though several relatives and supporters attended and sat at the back of the public gallery.

The Dickies, Nigel Baden-Clay and Olivia Walton are among 77 witnesses who could be called to give evidence during the four-week trial.

Other prospective witnesses include Queensland MP Bruce Flegg, a former neighbour of the Baden-Clays', as well as police officers, forensic and scientific experts and other neighbours.

COURT TOLD OF ALLISON BADEN-CLAY'S FINAL HOURS

Allison Baden-Clay's last day before she died was 'a day like any other Thursday' in her family household in the affluent western Brisbane suburb of Brookfield.

But as a court heard on Tuesday, beneath a veneer of normality trouble was brewing in the home where the former Miss Brisbane lived with her husband of almost 15 years, and cracks in their marriage were at breaking point.

In his opening address, Todd Fuller QC told the jury in the Brisbane Supreme Court on day one of the murder trial of Gerard Baden-Clay that he and his wife shared a bedroom, but that she sometimes slept on the living room couch or in another room.

It was April 19, 2012, and Allison, 43, and Gerard Baden-Clay were living with their three daughters, then aged 10, eight and five years old.

Allison had dropped her daughters at school and spent the day at a business meeting, preparing for a real estate conference the following day and organising a sleepover for her children and their friends.

She left the real estate agency at 4pm for a hair appointment.

Gerard Baden-Clay and his sister Olivia Walton went to watch his children at a cross country athletic event. At the sports field, Baden-Clay was 'bitten on the neck and had red marks on the side of his neck, but it's important,' Mr Fuller told the jury, 'he had no marks on the side of his neck'.

Nigel Baden-Clay picked up the girls while Gerard bought sausages for dinner. Meanwhile, Allison was texting and emailing her friend Kerry Anne Walker about the real estate conference.

Allison's final phone conversation was with her sister-in-law Olivia about the kids' sleepover.

Their three daughters went to bed between 6.30pm and 7pm. The eldest got up later for a drink of water and saw 'her mother in the lounge room watching TV' and her father 'dressed in the daggy shorts and t-shirt he usually wore to bed'.

The following morning, Gerard Baden-Clay contacted his father, police and his wife's parents, Priscilla and Geoff Dickie, to say that Allison was missing - perhaps after a morning walk.

He said he had last seen her at 10pm when he went to bed. Accounts would come out of Mrs Baden Clay's history of depression and anxiety about her marriage and the couple's financial problems.

When Gerard Baden-Clay was talking to police about his missing wife, they noticed scratches on his face which he said were 'shaving cuts'.

But a Doctor Kumar later examined Mr Baden-Clay and determined the marks were 'more consistent with scratches from a finger.'